This invention relates to injection molding, and more particularly, to an improved apparatus and method for facilitating space efficient injection molding using opposed nozzles.
The method of co-injection is characterized by injecting a first resin into a mold cavity, followed by, or simultaneously with, the injection with one or more other resin types into the same cavity. This method typically results in the article having multiple layers across its cross-section and generally a greater number of layers than resin types in the case where sequential injection of the resins is used.
The injection art includes a multitude of apparatuses and processes for forming molded articles from multiple resins by co-injection. For example, U.S. Pat. Nos. 5,028,226 and 4,717,324, show simultaneous and sequential co-injection apparatuses and methods, respectively. Both patents show one nozzle dedicated to a mold cavity wherein the mold cavity is completely filled for molding purposes by injecting a multitude of resins through a single gate orifice. Because a single orifice is used for the multitude of resins, the hot runner system employed to receive the various resin types from the resin source for conveyance to the molded cavities can be very complicated, as evidenced in the figures shown in the two patents. As a result, such complicated hot runner systems lead to mold designs which are not compact and thereby allow fewer cavities and fewer articles to be molded within a given space on a molding machine. Because of their complexity, the systems also require special design and manufacturing attention not normally required or encountered with less demanding one resin injection systems.
An alternative co-injection process is shown in U.S. Pat. No. 4,803,031, where molded articles are formed by injecting one resin into a cavity and filling the cavity space, and thereafter enlarging the cavity and filling the additional space with a second resin. The result of such a technique is two distinct areas each comprised of a resin type. However, such an arrangement requires two injection points into the mold cavity, resulting in complicated and bulky hot runner designs. As can be seen in the figures, the design of the hot runners dictate wide spacing of the cavities and are in themselves of a complicated non-rectilinear design. U.S. Pat. No. 4,275,030 suffers from these same problems, disclosing a similar method of producing a molded article from multiple resins. In this patent, the mold cavities are partitioned by movable blades, and resin is injected into a first partitioned portion of the cavity. The blade is then moved and resin is injected into the remaining portion of the cavity through an additional nozzle orifice. As a result, the different resin types are caused to join or weld at the area formally occupied by the blade.
U.S. Pat. No. 3,873,656 shows a co-injection apparatus wherein a multitude of plastics are injected into a mold cavity through a number of different gates. The timing of the opening and closing of the gates dictates the amount and extent of the resin types which form the molded article. As can be seen from the figures, the design is only suitable for molding very large plastic articles, wherein the multitude of gates can be used. Also, the hot runner shown does not have the capability for allowing individual temperature control of the different resin types, which inherently limits the variety of resins which can be used in the system together. In addition, since the gates are apart from one another, the flow of each resin will not be symmetrical throughout the part but instead will be biased in the area of the injection orifice.
U.S. Pat. No. 4,212,626 discloses an apparatus and method for transferring plastic in a stack mold arrangement. Two melt injection units are used, one unit is located in the stationary platen of the stack mold and the other unit is located in the movable platen of the stack mold. Each unit has its own runner passage and its own flow control gate. When the movable platen is in the closed position, against the stationary platen, the gates of each unit are in conducting communication. Accordingly, the gate of each unit is actuated to open and close when the platen is in the closed position in order to provide a flow of pressurized melt from the first runner passage to the second runner passage. The apparatus disclosed is not designed for use with two materials. One hot runner is provided for feeding a plurality of cavities from one nozzle. The hot runner passes through separate control valves and progressively moves plastic to each cavity. With only one hot runner providing materials to all cavities, only one material can be used to mold the articles.
U.S. Pat. No. 5,145,630 discloses a method and apparatus for improved core/cavity alignment in the production of a monomaterial thin walled container. This apparatus is used to inject plastic into the rim portion of a thick lipped article for stabilizing the position of the mold core with respect to the cavity during the process of plastic injection. In the embodiment shown in FIG. 4, simultaneous injection of plastic from opposite ends of the container is shown. The purpose of the simultaneous injection is apparently to stabilize the core shift by reducing plastic flow length through the thin wall section. As a result, more uniform flow is established. The apparatus disclosed is directed only for use with a single resin.
Hence, there exists a need in the injection molding art for a simplified mold design which allows for efficient positioning of nozzles and efficient space use, for the injection of a plurality of resins.